High-performance materials, particularly materials for bearing elements, such are used in turbochargers or exhaust gas recirculation systems for regulating flow in internal combustion engines for example, are known from the prior art. Various concepts are used, by which corresponding regulation systems are adjusted mechanically by externally positioned electric or pneumatic actuators using rod kinematics.
The penetration into the interior of the turbocharger or the exhaust gas recirculation system usually serves as the bearing for the adjustment as well. The bearing elements are in direct contact with the exhaust gas, with the result that temperatures on the bearing elements can reach as high as 950° C. Besides the high temperature load, the tribology and corrosion are also important as boundary conditions for the bearing elements.
With regard to tribological wear behaviour, the suitability of the material combination of the control system (shaft) and of the bearing material is of importance. Accordingly, different types of wear may occur, particularly adhesive or abrasive wear, tribochemical reaction wear or material fatigue. It is noticeable that in some material combinations the wearing effect decreases as the temperature rises. The oxide layers that are formed act as barrier layers, thereby preventing metallic contact between the tribological partners, which particularly reduces adhesive wear.
Furthermore, bearing elements in turbochargers and exhaust gas recirculation systems are exposed to environmental influences of varying severity depending on how they are installed in the engine compartment. In this context, corrosive phenomena play an important part. In particular, splashwater from the road, which in winter often contains road salt residues, can cause damage to surfaces. Besides these environmental and ambient sources of corrosive attack, exhaust gas condensates have been found to be the cause of corrosion more and more often in recent times. In particular, the use of exhaust gas recirculation systems has resulted in the increased incidence of corrosion by exhaust gas condensates. After the engine is stopped, localised condensation accumulates and is converted into hydrochloric, nitric or sulphuric acid by the chlorides and the nitrogen- or sulphur-bearing oxides in the atmosphere.
WO 2007/147710 A1 discloses a heat-resistant bearing material produced from an austenitic iron matrix alloy with a proportion of sulphur sufficient for achieving a solid lubricating action on the bearing surfaces thereof, and a proportion of 1 to 6% by weight of one or more of the alloying elements tungsten (W), cobalt (Co), niobium (Nb), rhenium (Re), molybdenum (Mo), tantalum (Ta), vanadium (V), hafnium (Hf), yttrium (Y), zirconium (Zr) and/or alloying elements having comparably high melting points. This material is used advantageously as a bearing material in turbochargers in combination with petrol engines due to the effective lubricating action of the sulphides together with the austenitic matrix and the good resistance thereof to creep, corrosion and oxidation. The disadvantage of this bearing material is that it is designed to be functionally reliable when used at high temperatures, preferably at temperatures above 600° C., and more preferably at temperatures above 850° C. This means that the properties of this material are most functionally effective at high temperature. On the other hand, in a temperature range below 400° C., such bearing materials are subject to greater wear.